1. Field of the Invention
The present invention relates to an image forming apparatus for exposing a photosensitive medium to light by way of a mask member.
2. Description of Related Art
Image recording apparatuses pertinent to the present invention are disclosed in U.S. Patent Applications Ser. Nos. 07/611,501 and 07/611,837 filed on Nov. 13, 1990. Both are assigned to the same assignee as that in the present application.
In the disclosed image recording apparatuses, the color image to be recorded is first separated into three colors, and each of the different color images is recorded onto a medium such as plain paper or a transparent sheet. The recording of the color images onto the medium is done by a monochromatic laser printer. The plurality of media on which the different color images have been respectively recorded are called mask members. Thus, the original color image is broken down into a plurality of mask members. Normally, three mask members for R (red), G (green) and B (blue) are formed.
In the subsequent color image recording operation where a copy of the original copy image is recorded on the plain paper or transparent sheet, a first mask member is fed onto a glass plate by rotation of plural rollers. Then, an exposure table, located above the glass plate, is lowered to an exposure position so that a photosensitive recording medium stretched below the exposure table, and which has not yet been exposed to light, is brought into close contact with the first mask member. The photosensitive recording medium, a photosensitive sheet consisting of a base sheet with microcapsules on one surface, is held to the exposure table by means of a fastening roller to prevent slippage of the photosensitive recording medium.
Each of the microcapsules contains a photosensitive substance, having a viscosity that is sensitive to irradiated light, and a chromogenic material that reacts when exposed to a developer to produce a color agent or dye. As the construction of the microcapsules is described in detail in the specification of U.S. Pat. No. 4,399,209, any further explanation thereof will be omitted herein.
While the photosensitive recording medium is in close contact with the first mask member, the first mask member is irradiated by light from a light source located below the first mask member, that is, below the glass plate. The light is of the color that produced the mask, that is red light is used if the mask member is the red one. Accordingly, the photosensitive recording medium is exposed according to the image recorded on the first mask member and a latent image of the recorded image is recorded on the photosensitive recording medium. After the photosensitive recording medium is exposed to the light, the exposure table is lifted to a retracted position to separate the photosensitive recording medium from the first mask member and the first mask member is replaced with a second mask member. Subsequently, the exposure sequence is repeated for all remaining mask members. Thus, the image from each mask member is exposed at the same portion on the photosensitive recording medium. As a result, a latent image of the original color image is recorded onto the photosensitive recording medium. The latent image on the photosensitive recording medium is then developed by developing means.
Since the recording medium is an elongated web drawn from a roll and taken up by a reel, it is necessary to absorb the change in path length of the photosensitive recording medium, associated with the vertical movement of the exposure table, a means for applying a tension to the photosensitive recording medium is provided in the image recording apparatus.
Further, in order to simultaneously carry out the exposure of the photosensitive recording medium by the exposing means and the development of the photosensitive recording medium after the exposure by the developing means, between the exposing means and the developing means of the image recording apparatuses there is a buffer means for temporarily storing an exposed part of the photosensitive recording medium. FIG. 5 shows the tension applying means and the buffer means provided in these conventional image recording apparatuses.
Referring to FIG. 5, reference numeral 40 generally designates a buffer device consisting of the buffer means and the tension applying means. The area of a photosensitive recording medium 38 in which a latent image of the color image has been formed by the exposure is temporarily stored in the buffer device 40. A pair of timing belts 65 are arranged in parallel to each other with each of the timing belts 65 wrapped around a pair of timing pulleys 66. A buffer roller 40a of the buffer device 40 is fixed at each of its ends to one of the timing belts 65. A step motor 67, or driving means, for driving the buffer device 40 is located on one side of the buffer device 40. A stator of the step motor 67 is rotatably supported to the apparatus frame or related structure and a rotor of the step motor 67 is fixed to the shaft of the timing pulley 66 on the one side of the buffer device 40.
An arm 68 projects from the stator of the step motor 67, and a free end portion of the arm 68 is opposed to a stopper 69. A spring 70, to apply tension, is connected between the apparatus frame and the arm 68, so as to resiliently rotate the stator of the step motor 67 in a clockwise direction, as viewed in FIG. 5, and thereby applying tension through the timing belts 65 and the buffer roller 40a to the photosensitive recording medium 38.
However, in these image recording apparatuses, the driving force applied to the buffer means is always constant. Therefore, when the exposure of the photosensitive recording medium 38 and the replacement of the mask member are carried out in association with the vertical movement of the exposure table, simultaneously with the development of a previously exposed portion of the photosensitive recording medium 38 wherein the previously exposed portion is drawn from the buffer means to the developing means, the driving speed of the step motor 67 for driving the buffer device 40 differs relative to the developing speed causing a fluctuation in the tension applied to the photosensitive recording medium 38.